netdev - Re: [PATCH bpf v2 1/2] bpf: mark registers as safe or unknown in all frames

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Message-ID: <6eb840f9-edd6-5bab-b824-75f35b5bca3b@iogearbox.net>
Date:   Tue, 23 Apr 2019 01:47:10 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Paul Chaignon <paul.chaignon@...nge.com>,
        Alexei Starovoitov <ast@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     Xiao Han <xiao.han@...nge.com>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH bpf v2 1/2] bpf: mark registers as safe or unknown in all
 frames

On 04/22/2019 08:34 PM, Paul Chaignon wrote:
> In case of a null check on a pointer inside a subprog, we should mark all
> registers with this pointer as either safe or unknown, in both the current
> and previous frames.  Currently, only spilled registers and registers in
> the current frame are marked.  This patch also marks registers in previous
> frames.
> 
> A good reproducer looks as follow:
> 
> 1: ptr = bpf_map_lookup_elem(map, &key);
> 2: ret = subprog(ptr) {
> 3:   return ptr != NULL;
> 4: }
> 5: if (ret)
> 6:   value = *ptr;
> 
> With the above, the verifier will complain on line 6 because it sees ptr
> as map_value_or_null despite the null check in subprog 1.
> 
> Note that this patch fixes another resulting bug when using
> bpf_sk_release():
> 
> 1: sk = bpf_sk_lookup_tcp(...);
> 2: subprog(sk) {
> 3:   if (sk)
> 4:     bpf_sk_release(sk);
> 5: }
> 6: if (!sk)
> 7:   return 0;
> 8: return 1;
> 
> In the above, mark_ptr_or_null_regs will warn on line 6 because it will
> try to free the reference state, even though it was already freed on
> line 3.
> 
> Fixes: f4d7e40a5b71 ("bpf: introduce function calls (verification)")
> Signed-off-by: Paul Chaignon <paul.chaignon@...nge.com>
> ---
>  kernel/bpf/verifier.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index db301e9b5295..777970d8c245 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -4876,11 +4876,11 @@ static void mark_ptr_or_null_regs(struct bpf_verifier_state *vstate, u32 regno,
>  		 */
>  		WARN_ON_ONCE(release_reference_state(state, id));
>  
> -	for (i = 0; i < MAX_BPF_REG; i++)
> -		mark_ptr_or_null_reg(state, &regs[i], id, is_null);
> -
>  	for (j = 0; j <= vstate->curframe; j++) {
>  		state = vstate->frame[j];
> +		for (i = 0; i < MAX_BPF_REG; i++)
> +			mark_ptr_or_null_reg(state, &state->regs[i], id,
> +					     is_null);

Small nit: I think it would be good to follow practice from clear_all_pkt_pointers()
and release_reg_references() to move handling of a singe frame into its own function.

>  		bpf_for_each_spilled_reg(i, state, reg) {
>  			if (!reg)
>  				continue;
> 

What about semantics in other, similar functions like find_good_pkt_pointers()?
Should this also consider all frames instead?

Thanks,
Daniel